Method for the production of bis(4-hydroxyaryl)-alkanes

ABSTRACT

A process for producing bis(4-hydroxyaryl)alkane is disclosed. The process comprise in sequence the steps of (a) passing inert gas through a melt at 150 to 230° C. that contains bis(4-hydroxyaryl)alkane and other aromatic hydroxy compounds, under conditions designed to remove said other aromatic hydroxy compounds from said melt and to obtain a stream of inert gas that contains said other aromatic hydroxy compounds (b) condensing the stream obtained in (a) to remove said other aromatic hydroxy compound, and to obtain a stream of inert gas, and (c) purifying and compressing the stream of inert gas obtained in (b), and recycling to step (a).

[0001] The present invention relates to a process for producingbis(4-hydroxy-aryl)alkanes from addition products ofbis(4-hydroxy-aryl)alkanes and aromatic hydroxy compounds which areobtained by the acid-catalysed reaction of aromatic hydroxy compoundswith ketones.

[0002] The synthesis of bis(4-hydroxyaryl)alkanes by the acid-catalysedreaction of aromatic hydroxy compounds with ketones is known from U.S.Pat. No. 2,775,620 or EP-A 342 758 for example. As a rule, an additionproduct of the bis(4-hydroxyaryl)alkane and of the aromatic hydroxycompound which is used as a starting material is obtained as anintermediate, and is subsequently freed from the aromatic hydroxycompound by distillation. The most important example of large-scaleindustrial production is the production of bisphenol A, during which anaddition product of bisphenol A (BPA) and phenol is obtained as anintermediate. Even after purification, by recrystallisation for example,this addition product still contains traces of acid (about 5 to 10.10⁻⁶mol acid/mol BPA) due to the acid-catalysed production route employed.On the separation of the phenol from bisphenol A, which is associatedwith an increase in temperature, these traces of acid result in thepartial decomposition of the bisphenol and in the formation ofby-products. The consequence of these decomposition reactions is animpairment of the purity and quality of colour of the bisphenol. Thisalso has a negative effect on the quality of products produced from thebisphenols, such as epoxy resins, polyesters, polyester carbonates andpolycarbonates, the consequences of which are problems of colour, poortransmission of light through transparent products, or pinholes in thesurfaces of mouldings produced from these end products. Similarphenomena also occur during the production of otherbis(4-hydroxyaryl)alkanes.

[0003] The separation of the aromatic hydroxy compound, which is alsotermed “stripping”, from the bis(4-hydroxyaryl)alkane, is known from theliterature and is described in EP-A 343 349 for example. The aromatichydroxy compound is separated from the bis(4-hydroxyaryl)alkane here byemploying steam in a packed column at 160 to 200° C. under a slightlyreduced pressure. It is also known, e.g. from U.S. Pat. No. 5,091,159,that thermal decomposition reactions occur during the separation of thearomatic hydroxy compound from the bis(4-hydroxyaryl)alkane.

[0004] A process has now been found which strongly suppressesdecomposition reactions during the stripping of the aromatic hydroxycompound (“monophenol”) from the bis(4-hydroxyaryl)-alkane. On theseparation of the monophenol from the bis(4-hydroxyaryl)alkane withinert gas in the course of this process, the recycled inert gas ispurified after the condensation of the monophenol, so that degradationof the bis(4-hydroxyaryl)alkane can be avoided.

[0005] The addition products of bis(4-hydroxyaryl)alkanes and aromatichydroxy compounds which can be used in the process according to theinvention can be obtained by the reaction of aromatic hydroxy compoundswhich are not substituted in the p-position and which contain no secondorder substituents such as cyano, carboxy or nitro groups, for examplephenol, o- and m-cresol, 2,6-dimethylphenol, o-tert.-butyl phenol,2-methyl-6-tert.-butylphenol, o-cyclohexylphenol, o-phenylphenol,o-isopropylphenol, 2-methyl-6-cyclopentyl-phenol, o- and m-chlorophenol,2,3,6-trimethylphenol, preferably phenol, o- and m-cresol,2,6-dimethylphenol, o-tert.-butylphenol and o-phenyl-phenol, mostpreferably phenol, with ketones which comprise at least one aliphaticgroup on their carbonyl function, for example acetone, methyl ethylketone, methyl propyl ketone, methyl isopropyl ketone, diethyl ketone,acetophenone, cyclohexanone, cyclopentanone, and methyl-, dimethyl- andtrimethylcyclohexanones which may also contain geminal methyl groupse.g. 3,3-di-methyl-5-methylcyclohexanone (hydroisophorone), preferablyacetone, acetophenone, cyclohexanone and homologues thereof whichcontain methyl groups, most preferably acetone. The addition product ofbisphenol A and phenol is preferably used as a starting material.

[0006] The present invention thus relates to a process for producingbis(4-hydroxyaryl)alkanes from addition products ofbis(4-hydroxyaryl)alkanes and aromatic hydroxy compounds, wherein

[0007] a) an inert gas is passed through a melt of an addition productof a bis(4-hydroxyaryl)alkane and an aromatic hydroxy compound at 150°C. to 230° C., wherein the inert gas stream removes the aromatic hydroxycompound from the melt,

[0008] b) the aromatic hydroxy compound is removed from the inert gasstream by condensation,

[0009] c) the inert gas stream is purified, compressed and recycled tostep a).

[0010] Separation of the monophenol from the bis(4-hydroxyaryl)alkanefrom the melt of the addition product of bis(4-hydroxyaryl)alkane andaromatic hydroxy compound is effected at temperatures between 150° C.and 230° C., preferably from 170° C. to 210° C., by expelling themonophenol by the introduction of an inert gas (e.g. nitrogen). Theratio of gas to the amount of addition product is preferably about 10 m³to 1000 m³ per tonne of addition product. Stripping can optionally befacilitated by reducing the pressure in the processing unit which isused for the separation of the monophenol, but is preferably effectedunder normal pressure. Stripping of the monophenol by the inert gas isconducted in known apparatuses, for example in a packed, flooded column.The monophenol can be removed from the inert gas circulation system, forexample, by condensation on a heat exchanger.

[0011] Purification of the recirculated inert gas can be effected bypassing it over a fixed bed adsorbent (activated carbon, zeolite etc.),or in one preferred embodiment can be effected by intensively scrubbingthe inert gas. This scrubbing step can be effected in a gas scrubber forexample. In one preferred embodiment, during the compression of theinert gas of the circulating flow in a compressor the sealing liquidwhich is employed there is used as the scrubbing medium.

[0012] A slightly alkaline aqueous solution is used as the scrubbingmedium. The pH of the aqueous solution which is used should fall withinthe range from 7 to 12, preferably from 7.5 to 11, most preferably from8 to 10. All substances which exhibit a basic effect can be used for theproduction of an aqueous solution such as this. Alkali and alkalineearth hydroxides are preferred. In one preferred embodiment, the pH ismonitored before and after scrubbing, so that excessive and insufficientadditions are prevented during the purification of the circulating gascircuit. Moreover, the inert gas stream can subsequently be fed to asecond scrubbing step. The second scrubbing medium preferably has aneutral pH.

[0013] The bis(4-hydroxyaryl)alkanes which are produced by the processaccording to the invention are distinguished by their very good inherentcolour and by their high purity. In particular, they comprise lowcontents of aromatic hydroxy compounds (<100 ppm, preferably <50 ppm)and of decomposition products (e.g. isopropenylphenol, dimericisopropenylphenol).

[0014] Polymers such as polycarbonates or epoxides which exhibit a lowdegree of inherent colour can be manufactured frombis(4-hydroxyaryl)-alkanes which are produced by the process accordingto the invention.

EXAMPLES Example 1

[0015] A molten mixture of BPA and phenol (60/40% by weight) was fedinto a desorber at a rate of 1 tonne/hour. 225 m³/hour of nitrogen (as acirculating flow) were passed continuously into the desorber. The phenolwas taken up by the nitrogen and was subsequently condensed by means ofa heat exchanger. The nitrogen was subsequently fed to a compressor, thesealing liquid of which was acted upon by very dilute NaOH solution (pH10), where the nitrogen was compressed. The nitrogen was then introducedinto a gas scrubber which was operated using deionized water. Thenitrogen which was treated in this manner was then reused in thedesorber for the stripping of phenol. A low-phenol (phenol content 40ppm), light-coloured BPA melt was then obtained, which had a melt Hazencolour of 8.

Comparative Example 1

[0016] The procedure was as in Example 1, except that scrubbing of thenitrogen was completely omitted. A BPA was then obtained which had aHazen colour of 17. The phenol content of the BPA was 90 ppm, whichindicated decomposition.

Comparative Example 2

[0017] The procedure was as in Example 1, except that neutral deionisedwater (pH 6.9) was used for scrubbing the nitrogen. A BPA was thenobtained which had a Hazen colour of 14. The phenol content of the BPAwas 75 ppm, which again indicated decomposition.

Comparative Example 3

[0018] The procedure was as in Example 1, except that a dilute NaOHsolution (pH 13) was used for scrubbing the nitrogen. A BPA was thenobtained which had a Hazen colour of 12. The phenol content of the BPAwas 60 ppm, which still indicated decomposition.

1. A process for producing bis(4-hydroxyaryl)alkanes from additionproducts of bis(4-hydroxyaryl)alkanes and aromatic hydroxy compounds,wherein a) an inert gas is passed through a melt of an addition productof a bis(4-hydroxy-aryl)alkane and an aromatic hydroxy compound at 150°C. to 230° C., wherein the inert gas stream removes the aromatic hydroxycompound from the melt, b) the aromatic hydroxy compound is removed fromthe inert gas stream by condensation, c) the inert gas stream ispurified, compressed and recycled to step a).
 2. A process according toclaim 1, wherein purification of the inert gas stream is effected byscrubbing with an aqueous medium with a pH within the range from 7 to12.
 3. A process according to claim 2, wherein the sealing liquid of thecompressor used for the compression of the inert gas is employed as thescrubbing medium.
 4. A process according to claims 2 or 3, wherein afteralkaline scrubbing and before it is recycled to step a) the inert gasstream is subjected to a second scrubbing step in an aqueous medium ofneutral pH.
 5. A process according to claim 1, wherein purification ofthe inert gas stream is effected by passing it over a fixed bedadsorbent.
 6. A process according to claim 5, wherein activated carbonor zeolite is used as the fixed bed adsorbent.
 7. Bis(4-hydroxy)alkaneswith a content of aromatic hydroxy compounds of <50 ppm, obtainable by aprocess according to claim 1.